1. Field of the Invention
This invention relates to improvements in the vacuum casting apparatus in which a gas-tight chamber for accommodating a die is evacuated to evacuate a die cavity so as to lead molten metal through a stalk into the evacuated cavity.
2. Description of the Prior Art
In the above vacuum casting apparatus, a melting furnace for storing molten metal in its molten state is disposed outside the gas-tight chamber and connected to the cavity via the stalk. To this end, the wall of the gas-tight chamber has a through hole through which the stalk is passed. In order to be able to obtain satisfactory evacuation in the gas-tight chamber, it is necessary to provide satisfactory seal of the gap between the surface of the through hole and the stalk. To meet this requirement, many techniques have been proposed.
The most well-known one of these techniques is a system in which the gap is sealed with a flange extending from the outer periphery of the stalk. A contrivance for obtaining a better seal with such a flange is disclosed in Japanese Laid-Open Patent Publication No. 63-84757. FIG. 20 shows this technique. In this technique, a lower wall member 511 is detachably mounted on the underside of a gas-tight chamber 502 via a seal 512. A sub-chamber 514 is thus formed by the gas-tight chamber 502 and the lower wall member 511, and is communicated with the gas-tight chamber 502 via a through hole 522. The gap between the lower wall member 511 and a stalk 505 is sealed with a flange 505a formed on the outer periphery of the stalk 505. In this system, the sub-chamber 514 is evacuated, and thus it is possible to hold the amount of air intruding through the gap between the surface of the through hole 502b of the gas-tight chamber 502 and the stalk 505 to be low.
This system is an effective technique insofar as obtaining satisfactory gas-tight chamber evacuation. However, it is necessary to provide the flange 505a on the outer periphery of the stalk 505.
The stalk should not be one which contaminates the molten metal by releasing impurities thereinto when exposed to the high temperature of the molten metal. Accordingly, it has been contemplated to form the stalk with the same metal as the molten metal. Where the stalk and the molten metal are of the same metal, the problem of the contamination of molten metal is not posed.
Where the stalk is made of the same metal as the molten metal, it is readily melted. Therefore, it has to be replaced frequently, although the used stalk can be used as the material of the molten metal and does not lead to any waste of material. However, since the stalk is replaced frequently, it is required to form the stalk inexpensively.
Heretofore, a flange has been provided on the outer periphery of the stalk to permit improvement of the seal. Such a flange, however, dictates a cumbersome step of its formation. A problem is thus posed in connection with the cost of the flange formation on the stalk. In this background, there has been a demand for a technique for obtaining satisfactory seal of the gap with a flange-free stalk (hereinafter referred to as straight-tubular stalk).
A technique which is disclosed in Japanese Laid-Open Patent Publication No. 2-284962 will now be described with reference to FIG. 18.
FIG. 18 shows a vacuum casting apparatus 402 which mainly comprises a die 404 made of sand and a gas-tight surface plate 420 on which the die 404 rests. The die 404 comprises an upper and a lower die half 406 and 408 overlapped over each other to form an inner cavity 410 having a shape complementary to the product shape.
Gas tightness means 418 is provided on the upper surface of the gas-tight surface plate 420. The gas tightness means 418 is constituted by a member which, when the die 404 is set on the gas-tight surface plate 420, is pushed into the lower die half 408 made of sand to push a portion of the lower die half 408 surrounding a sprue 416 toward the center thereof.
In the vacuum casting using the vacuum casting apparatus 402, first the upper and lower die halves 406 and 408 of the die 404 are assembled together with a frame member 412 secured to their portions with the intervening seam therebetween. Then, a straight-tubular stalk 430 is inserted into the sprue 416 provided on the underside of the die 404 assembled in the above way.
Then, the die 404 is set on the gas-tight surface plate 420 with the stalk 430 inserted therethrough. At this time, the gas tightness means 418 provided on the gas-tight surface plate 420 is pushed into the lower die half 408. As a result, the wall of the sprue 416 is pushed against the outer periphery of the stalk 430, so that the gas tightness between the stalk 430 and the sprue 416 is secured.
Further, stalk securing means 422 provided on the underside of the gas-tight surface plate 420 is tightened by an air cylinder mechanism 424 to secure the stalk 430 to the gas-tight surface plate 420. Thereafter, a gas-tight chamber 414 is mounted to enclose the die 404, and its portion in contact with the gas-tight surface plate 420 is sealed, thus forming a gas-tight space surrounding the die 404.
In this state, the lower end of the stalk 430 is dipped in molten metal M40, and the pressure in the gas-tight chamber 414 is reduced by a vacuum pump 434 via a vacuum tubing 432 connected to the gas-tight surface plate 420. Thus, the molten metal M40 is withdrawn into the stalk 430 and fills the cavity 410. In this way, vacuum casting can be performed using the straight-tubular stalk 430 having no flange thereon.
In the above vacuum casting apparatus 402, the gas tightness means 418 is provided to obtain close contact between the sprue 416 of the die 404 and the stalk 430. However, where the die 404 of sand is formed by making use of chemical hardening based on a binder, it can be deformed very slightly, so that it is difficult to obtain close contact of the inner wall of the sprue 416 with the outer periphery of the stalk 430.
Therefore, it is impossible to ensure sufficient gas tightness, and external air penetrated through the gap between the stalk 430 and the die 404 is introduced into the cavity 410 at the time of the evacuation, thus resulting in the formation of pores or voids in the casting to deteriorate the characteristics of the product. That is, a problem is posed that excellent casting quality which is a feature of the vacuum casting process can not be obtained. Further, with a deformable sand die made of raw sand or the like, falling of sand, deformation of unnecessary portion, etc., are liable to take place at the time of the deformation by the gas tightness means 418.
Another technique which also uses a straight-tubular stalk is disclosed in Japanese Laid-Open Patent Publication No. 4-294854. This technique will now be described with reference to FIG. 19.
Referring to FIG. 19, designated at 450 is a die, at 452 a gas-tight surface plate, at 460 a straight-tubular stalk, and at 430 a flexible seal member. Designated at 440A and 440B are seal retainer halves like ring halves having a tapered outer periphery. The gas-tight surface plate 452 has a through hole 452a which has a complementary taper shape.
With this arrangement, when the seal retainer halves 440A and 440B are pushed down by the die 450 set on the gas-tight surface plate 452, their taper surfaces compress the seal member 430. As a result, the seal between the gas-tight surface plate 452 and the stalk 460 is enhanced. The gap between the gas-tight surface plate 452 and the stalk 460 thus can be sealed well, thus considerably alleviating the inconvenience of void formation or the like due to trapping of bubbles in the casting. However, the seal obtainable with this system is inferior to the seal obtainable with the system using a stalk with a flange.